The present invention is directed to dewatering screen devices and in particular to such screen devices utilizing a pressurized feed input.
Dewatering screen devices are well known and comprise screen surfaces mounted in housings to which a liquid-solids slurry is fed for separation of the solids from the liquids on the screen surface. In one type of dewatering screen device the screen comprises a highly polished concave curved screen surface or deck having an arcuate configuration formed to a configuration of a 120.degree. arc segment of a circle. The screen surface is comprised of a plurality of equidistantly spaced and parallel transverse separating bars having slot openings of 50 to 150 micron size range formed therebetween. The slurry is fed in a substantially tangential direction to the concave side of the screen surface at a relatively high pressure through spaced feed nozzles. Oversize material travels around the screen surface and is discharged from the device through an overflow outlet pipe. The liquid portion of the slurry together with fines pass through the slots in the screen surface and are discharged through an underflow outlet at the rear of the screen.
These dewatering screens have met with wide success and are essentially free from maintenance cost in view of the simplicity of the design and minimum number of moving parts. In particular the screens are extensively used for high capacity fine separation of fibrous non-abrasive solids in the pulp and paper industry where the screens are used for many process purposes, such as for example, as savealls or in fiber recovery. Although the screens are highly effective, inconveniences are present in certain applications in view of known feed nozzle arrangements. The most common nozzle arrangements for these pressure feed devices comprise a plurality of nozzles attached to the upper portion of the unit housing at spaced intervals for dispersing the feed to the screen surface. In operation of the screen unit the orifices of the nozzles will intermittently clog requiring the interruption of the operation of the unit for cleaning. Further, although the nozzles are effective in use they often result in a lack of full utilization of the screen surface due to the spacing of the nozzles which causes intermediate blind areas across the screen. This is especially noticeable at the opposite sides of the screen where a splashing effect is generated which result in a loss of separation effectiveness in those areas of operation.
In other known dewatering screen devices such as for example those described and disclosed in British Pat. No. 829,860 and U.S. Pat. Nos. 2,995,245; 3,353,674 and 3,452,876 feed inlet arrangements for screen surfaces are disclosed which comprise movable flaps or displaceable wall surfaces which direct the flow onto the screen. The wall surfaces are adjustable either by the flow of the material or manually to vary the distance between the wall surface and the screen. These arrangements although sufficient for their intended purposes are not concerned with controlling the velocity of the feed flow to the screen surface at relatively high pressure feed rates.
It is an object of the present invention to provide a novel flow distribution means for a dewatering screen.
Another object is to provide a novel nozzle and feed spout arrangement for a liquid-solids separating screen which allows for full utilization of the separating screen surface of the unit.
Another object is to provide a novel nozzle and feed spout for dewatering screen unit having readily operable means for opening the feed spout to permit cleaning of the nozzle assembly upon clogging thereof.
Another object is to provide a novel dewatering screen which allows for rapid purging of the nozzle assembly in case of clogging and which includes means for controlling the velocity of the feed during purging.
A further object is to provide a novel nozzle assembly having means for adjusting the size of the spout opening through infinitely variable settings in accordance with the flow of the feed slurry.
A still further object is to provide a novel nozzle and spout arrangement for dewatering screen which is adapted for automatic operation for selective intermittent opening and closing of the spout to assure proper functioning of the nozzle during operation of the dewatering screen.